Well logging analysis methods for use in complex lithology reservoirs

Abstract

Advanced analytic methods and formulas have been developed for 2- and 3- mineral solutions in complex lithology reservoirs. Required logs are density, neutron, acoustic, and photoelectric absorption index (PE curve). On the basis of developed mathematic models of complex reservoirs the limitations and maximum possible errors in the determination of porosities and lithologic compositions are presented for each method. In most cases, the formulas developed are simple enough to be used with programmable calculators.

I claim: 
     
       1. A machine implemented iterative method of well log analysis to determine the volumes of anhydrite (or gypsum) in three mineral combination lithology utilizing well log measurements of gamma-gamma density, neutron and acoustic properties of earth formations in the vicinity of a well borehole comprising the steps of: (a) obtaining a gamma-gamma density versus neutron porosity crossplot from the respective logs at a corresponding depth in the well borehole;   (b) determining ρ map  (the average density of rock matrix) and Δt map  (the average matrix acoustic travel time) by solving said crossplot for a two mineral lithology;   using the ρ map  and Δt map , then determining the volume of anhydrite (or gypsum) present from the relationship given by   (ρ.sub.map (1-Va)+ρ.sub.a ·V.sub.a -ρ.sub.b)/(ρ.sub.map -ρ.sub.ef)=[Δt-(Δt.sub.map)(1-V.sub.a)+Δt.sub.a ·V.sub.a)]/(Δt.sub.f -Δt.sub.map)        where V a  is the volume of anhydrite in the formation; ρ a  is the density of anhydrite; ρ b  is the bulk density from the density log response; ρ ef   is the electron density of the formation fluid; Δt is the acoustic travel time from the acoustic log response; Δt a  is the acoustic travel time of anhydrite; and Δt f  is the acoustic travel time in the formation fluid;   (c) using the V a  just computed, recompute the two mineral lithology volume for V l  (volume of limestone) from the relationship   V.sub.l =V.sub.l ·(1-V.sub.a),        and;   repeating steps (b) and (c) using new values of ρ map  and Δt map  in an interative manner until the corrected values of V a  do not vary from repetition to repetition by more than a predetermined amount, thereby arriving at a value of V a .   
     
     
       2. A machine implemented method of well log analysis to determine earth formation properties of porosity, secondary porosity and the volumes of sandstone, limestone, dolomite, gypsum and anhydrite in three mineral combination lithology utilizing gamma-gamma density, neutron, acoustic, and photoelectric absorption index Pe, properties of earth formations in the vicinity of a well borehole, comprising the steps of: (a) obtaining in a well borehole gamma-gamma density, neutron, acoustic and photoelectric absorption index Pe measurements of earth formation properties in the vicinity of the well borehole;   (b) using a gamma-gamma density versus neutron porosity crossplot determine the possible maximum and minimum values for dolomite, limestone and sandstone;   (c) on the basis of the computed lithological composition determine the porosity as in a two mineral solution;   (d) determine U (photoelectric absorption index per unit volume) from the measured Pe and ρ b  (bulk density);   (e) determine U e  and U d  and U s  (photoelectric absorption index per unit volume for limestone, dolomite and sand) from porosity and known U values;   (f) for each pair of minerals determine maximum and minimum values of U and determine the volume of dolomite or sand V d  or V s  ;   (g) correct the neutron log response for the presence of dolomite (or sand) using the relationship ##EQU13##  where φ n  is the measured neutron porosity response: φ nd  is the neutron log response to dolomite;   (h) determine the volume fraction of limestone from the relationship ##EQU14##  where φ ns  is the neutron log response to sandstone and φ nl  is the neutron log response to limestone; (i) determine the volume fraction of sandstone V s  from the relationship   V.sub.s =1-V.sub.l -V.sub.d.         
     
     
       3. A machine implemented method of well log analysis for determining the density of hydrocarbons in a complex reservoir in earth formations penetrated by a well borehole using density, neutron and Pe photoelectric absorption index measurements made in the borehole, comprising the steps of: (a) obtaining well log measurements in a well borehole of the response of reservoir rocks to gamma-gamma density, neutron and photoelectric absorption index measuring systems;   (b) determine the voluem of limestone V l  and dolomite V d  in the reservoir from the relationship   U=V.sub.l ·V.sub.l +(1-V.sub.l)·U.sub.d where       V.sub.d =1-V.sub.l        U is the measured photoelectric absorption index; U l  is the measuring system response in pure limestone; U d  is the measuring system response in pure dolomite;   (c) determine the density ρ bw  and neutron φ nw  log responses for this composition reservoirs from the relationships   φ.sub.nw =V.sub.l ·φ.sub.nl +V.sub.d ·φ.sub.nd       ρ.sub.bw =V.sub.l ·φ.sub.bl +V.sub.d ·φ.sub.bd        where φ nl  is the neutron log response in limestone   φ nd  is the neutron log response in dolomite;   ρ bl  is the density log response in limestone;   ρ bd  is the density log response in dolomite;     (d) determine the neutron log response φ nw  to a water or response φ nh  to a hydrocarbon bearing reservoir using the relationship   φ.sub.nw =φ.sub.e +φ.sub.nm       φ.sub.nh =φ.sub.e ·Hl.sub.f +φ.sub.nm        where φ e  is the effective porosity;   HI f  is the hydrogen index of the fluid; and φ nm  is the neutron log response to a rock matrix and where   φ.sub.nw -φ.sub.nh =φ.sub.e (1-HI.sub.f)       HI.sub.f =1-(φ.sub.nw -φ.sub.nh)/φ.sub.e         (e) similarly, develop the density log responses ρ bw  (response to water bearing reservoir), ρ bh  (response to hydrocarbon bearing reservoir) and ρ ef  (electron density of fluid) using the relationships   ρ.sub.bw =ρ.sub.ma (1-φ.sub.e)+φ.sub.e ·ρ.sub.w       ρ.sub.bh =ρma(1-φ.sub.e)+φ.sub.e ·ρ.sub.ef       ρ.sub.ef =ρ.sub.w -(ρ.sub.bw -ρ.sub.bh)/φ.sub.e        where ρ w  is the density of water and the other terms are as defined;   (f) determine from resistivity mesurements the water saturation S w  of the reservoir and then determine from the following relations HI h  (the hydrogen index of hydrocarbons in the reservoir) and ρ eh  (the electron density of the reservoir hydrocarbons)   HI.sub.h =(HI.sub.f -S.sub.w)/(1-S.sub.w)       ρ.sub.eh =(ρ.sub.ef -S.sub.2 ·ρ.sub.w)/(1-S.sub.w).